The present invention relates to an electrophotographic type color image forming apparatus having an endless intermediate transfer member, and particularly to effective technology applied to prolongation of a photoconductor's life in a color image forming apparatus.
Conventionally, in an image forming apparatus adopting an electrophotographic form, a photoconductor that is an image carrying member is charged by a charger, the charged photoconductor is irradiated with light according to image data to form a latent image thereon, this latent image is developed by a developing device, and the developed toner image is transferred onto a recording medium, whereby an image is formed.
On the other hand, with colorization of image, a tandem type color image forming apparatus is also proposed, in which plural image forming units executing such each image forming process are provided, toner images of cyan, magenta, yellow, and preferably black are formed on the respective photoconductors, and these toner images are multi layer transferred onto an endless intermediate transfer member at a transfer position of each photoconductor, whereby a full color image is formed.
Since such the tandem type color image forming apparatus has image forming sections for each color, speed-up is advantageous.
A conventional tandem type color image forming apparatus will be described below.
FIG. 6 is a schematic view showing the constitution of the conventional color image forming apparatus 100.
In FIG. 6, in a body 1 of the color image forming apparatus, image forming units 102, 103, 104 and 105 for forming respectively toner images of each color of yellow (Y), magenta (M), cyan (C), and black (K) are arranged, and exposing devices 106a, 106b, 106c, and 10d respectively corresponding to the image forming units 102 to 105 are provided. The image forming units 102 to 105 include photoconductor drums (photoconductors) 102a, 103a, 104a, and 105a on peripheral surfaces of which electrostatic latent images are formed by irradiation of laser beams from the exposing devices 106a to 106d; and developing rollers 102b, 103b, 104b, and 105b which attach toner supplied from toner tanks onto the photoconductor drums 102a to 105a to make the electrostatic latent images visible as toner images.
On the lower side of the image forming units 102 to 105, an endless intermediate transfer belt (intermediate transfer member) 107 on which a color toner image is formed by multi layer transferring the toner images of the respective colors made visible on the photoconductor drums 102a, 103a, 104a, and 105a is arranged so that it can run in the direction of an arrow. The intermediate transfer belt 107 includes a drive roller 108, a tension roller 109, four first bias transfer rollers 110a, 110b, 110c and 110d, and a driven roller 111 in its loop.
At the lower portion of the apparatus, a paper supply cassette 113 storing paper (recording medium) P therein is provided. The paper P is fed out from the paper supply cassette 113 to a paper transport path one by one by a supply roller.
On the paper transport path, a second bias transfer roller 112 which comes into contact with the peripheral surface of the intermediate transfer belt 107 by the predetermined amount in the position of the driven roller 111 thereby to transfer the color image on the intermediate transfer belt 107 onto the paper P, and a fixing device 114 which fixes the color image transferred onto the paper P onto the paper P are arranged.
In the thus structured image forming apparatus, onto the surface of the intermediate transfer belt 107, the toner images of yellow, magenta, cyan and black adhere from the photoconductor drums 102a to 105a in the image forming units 102 to 105 and the color image is formed. This toner color image is transferred onto the paper P taken out of the sheet supply cassette 113 by the nip power between the driven roller 111 and the second bias transfer roller 112. Then, the paper P is supplied to the fixing device 114, the toner image is fixed thereon, and thereafter the paper P is exhausted.
In such the color image forming apparatus, the first bias transfer rollers 110a to 110d move to two positions up and down. One of their positions is a contact position where the first bias transfer rollers bring the intermediate transfer belt 107 into pressure contact with the photoconductor drums 102a to 105a, and the other is a separation position where the first bias transfer rollers separate the intermediate transfer belt 107 from the photoconductor drums 102a to 105a. The image forming units 102 to 105 stop after the first bias transfer rollers 110a to 110d have moved downward and the intermediate transfer belt 107 and the photoconductor drums 102a to 105a have separated from each other.
Here, when the intermediate transfer belt 107 and the photoconductor drums 102a to 105a are separating from each other, the state of the intermediate transfer belt 107 changes. Therefore, when the toner image on the intermediate transfer belt 7 is transferred onto paper P for this time, transfer error occurs. In order to prevent this transfer error, after the toner image on the intermediate transfer belt 107 has been transferred onto the paper P, the first bias transfer rollers 110a to 110d move downward, contact between the photoconductor drums 102a to 105a and the intermediate transfer belt 107 is released, and rotation drive of the image forming units 102 to 105 including the photoconductor drums 102a to 105a stops.
However, in the described-before related art, after the toner image on the intermediate transfer belt 107 has been transferred on the paper P, the separation between the photoconductor drums 102a to 105a and the intermediate transfer belt 107 is performed. Therefore, though the toner images on the photoconductor drums 102a to 105a have been already transferred on the intermediate transfer belt 107, the photoconductor drums 102a to 105a come into contact with the intermediate transfer belt 107. Further, the photoconductor drums 102a to 105a in the image forming units 102 to 105 and the developing units 102b to 105b are operating. Therefore, there is a problem that lives of the photoconductor drums 102a to 105a are shortened.
As described before, when the intermediate transfer belt 107 and the photoconductor drums 102a to 105a separate from each other, the state of the intermediate transfer belt 107 changes and transfer error occurs. Therefore, it is not possible to unnecessarily separate the intermediate transfer belt 107 and the photoconductor drums 2a to 5a from each other during printing.
Here, a lifting mechanism of the first bias transfer rollers 110a to 110d which the inventor has been investigated will be shown in FIG. 7.
FIG. 7A shows a state where all the first bias transfer rollers 110a to 110d descend and the intermediate transfer belt 107 separate from the photoconductor drums 102a to 105a. 
When an instruction is given so that only the black image forming unit 105 operates in case that a monochrome image is formed, a drive motor (not shown) operates, and a second transmission shaft 117 is rotated. Then, as shown in FIG. 7B, a rise and fall rod 118c rises by a rise and fall cam 117a, and the first bias transfer roller 110d attached to this rise and fall rod 118c brings the intermediate transfer belt 107 into contact with the photoconductor drum 105a. 
Further, when an instruction is given so that all the image forming units 102 to 105 operate in case that a color image is formed, first and second transmission shafts 116 and 117 are rotated. Then, as shown in FIG. 7C, rise and fall rods 118b rise by a rise and fall cam 116a through a horizontal rod 118a, and the rise and fall rod 118c rises by the rise and fall cam 117a, so that the first bias transfer rollers 110a to 110d attached to the rise and fall rods 118b and 118c bring the intermediate transfer belt 107 into contact with the photoconductor drums 102a to 105a. 
In order to appropriately transfer a toner image formed on a photoconductor drum onto an intermediate transfer belt, it is necessary to bring the intermediate transfer belt into pressure contact with the photoconductor drum by the optimum contact power of a first bias transfer roller.
However, in the structure in which the first bias transfer roller is raised and let down by the before-mentioned link mechanism, the number of parts increases and accuracy is difficult to obtain, so that it is difficult to attempt optimization. Namely, since the number of elements for accuracy acquirement such as machining accuracy of the rise-and-fall cam and the transmission shaft, and a rotary fulcrum increases, it is difficult to bring the intermediate transfer belt into pressure contact with the photoconductor drum with the optimum contact power by the first bias transfer roller.